A: Operating phase shifters outside of their specified frequency range, particularly at the low frequency end, is difficult because you run out of electrical length.
An option for phase shifter of lower frequency range is ADL5390, that can be used as wideband vector modulator (20 MHz-2.4 GHz). It does require an external 90 deg phase splitter to be made operational. Also, the phase is controlled by two analog voltages, an I control voltage and a Q control voltage. If digital phase shifter is needed, additional two DACs are required.
